Actuators are used for a variety of purposes. Relays are a common example. Typically, actuators include an excited member and a passive member. The excited member normally acts upon the passive member to move, and sometimes control the position of, the passive member. In the case of a relay, a movable member (commonly the passive member) is moved relative to the fixed member, which is commonly the excited member. Electromagnetic relays operate by pulling the passive member toward the excited member using electromagnetic energy. The passive member is usually returned with a mechanical urging force, such as accomplished with a spring.
There are also myriad situations in which actuators are used to control the relative positions of the actuator members in a number of degrees of freedom. Magnetic bearings, and inertial instruments such as gyroscopes and accelerometers, are examples. In such cases, the position of a spinning shaft or wheel must be precisely controlled. Control movement of the shaft or wheel is accomplished through electromagnetic, or a combination of magnetic and electromagnetic, energy, typically provided by the stator.
One goal of actuator development is miniaturization. In inertial instruments, a smaller and lighter rotor requires less force to suspend and control its position. This reduces the power required to operate the device. Another goal is to increase the developed force relative to the rotor mass, which allows the device to withstand higher acceleration forces. In relays, such higher force relative to armature (passive member) mass leads to greater actuation rates.